A helium atom is smaller than a hydrogen atom. Although helium
has one more electron, it also has an additional positive charge
on the nucleus that pulls both electrons closer to the nucleus.
Atoms of the second-shell elements Li through Ne are larger than
either hydrogen or helium because the second electron shell is farther
from the nucleus. Also, there is a gradual decrease in atomic
size from Li to Ne. The inner two electrons shield the nucleus,
so the outer electrons experience the pull from a "helium core"
with a net positive charge that is two less than the actual atomic
The lone outer electron in Li feels a net central charge of +3 -
2 = +1. The four outer electrons in C experience a central attraction
of +6 - 2 = +4, and the seven electrons around F experience a charge
attraction of +7. As the central charge increases with atomic number
from Li to Ne, the outer electrons are drawn more tightly toward
the nucleus by electrostatic attraction, thereby causing the atoms
to be successively smaller. This has an important effect on their